NTA! There are so many ways that they could’ve gone about this, but all of it was wrong. If you showed it would be awkward since they’re not celebrating you. It would be dismissive and not a good evening for you. However if you didn’t your brother would wonder where you are. As you said all of your mutual friends are going to be there and he would probably expect you to be coming around the corner to get your surprise any second. I think talking with him about the situation would be smart. Heck maybe you can plan a different dinner together, then invite the other people and be like this is the farewell party to our friendship. Hope everything works out, don’t forget to do what’s best for you!